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Thomas G. Lockhart authoredThomas G. Lockhart authored
set.sgml 21.95 KiB
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/set.sgml,v 1.30 2000/02/19 08:14:54 thomas Exp $
Postgres documentation
-->
<refentry id="SQL-SET">
<refmeta>
<refentrytitle id="SQL-SET-TITLE">
SET
</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
SET
</refname>
<refpurpose>
Set run-time parameters for session
</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
SET <replaceable class="PARAMETER">variable</replaceable> { TO | = } { '<replaceable class="PARAMETER">value</replaceable>' | DEFAULT }
SET TIME ZONE { '<replaceable class="PARAMETER">timezone</replaceable>' | LOCAL | DEFAULT }
SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
</synopsis>
<refsect2 id="R2-SQL-SET-1">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">variable</replaceable></term>
<listitem>
<para>
Settable global parameter.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">value</replaceable></term>
<listitem>
<para>
New value of parameter. <option>DEFAULT</option> can be
used to specify resetting the parameter to its default value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The possible variables and allowed values are:
<variablelist>
<varlistentry>
<term>CLIENT_ENCODING | NAMES</term>
<listitem>
<para>
Sets the multi-byte client encoding. Parameters are:
<variablelist>
<varlistentry>
<term><replaceable class="parameter">value</replaceable></term>
<listitem>
<para>
Sets the multi-byte client encoding to
<replaceable class="parameter">value</replaceable>.
The specified encoding must be supported by the backend.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
This option is only available if MULTIBYTE support was enabled
during the configure step of building Postgres.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DateStyle</term>
<listitem>
<para>
Set the date/time representation style. Affects the output format,
and in some cases it can affect the interpretation of input.
<variablelist>
<varlistentry>
<term>ISO</term>
<listitem>
<para>
use ISO 8601-style dates and times
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SQL</term>
<listitem>
<para>
use Oracle/Ingres-style dates and times
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Postgres</term>
<listitem>
<para>
use traditional <productname>Postgres</productname> format
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>European</term>
<listitem>
<para>
use dd/mm/yyyy for numeric date representations.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>NonEuropean</term>
<listitem>
<para>
use mm/dd/yyyy for numeric date representations.
</para>
</listitem>
</varlistentry>
<varlistentry> <term>German</term>
<listitem>
<para>
use dd.mm.yyyy for numeric date representations.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>US</term>
<listitem>
<para>
same as 'NonEuropean'
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DEFAULT</term>
<listitem>
<para>
restores the default values ('US,Postgres')
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Date format initialization may be done by:
<simplelist>
<member>
Setting the <envar>PGDATESTYLE</envar> environment variable.
If PGDATESTYLE is set in the frontend environment of a client
based on libpq, libpq will automatically set DATESTYLE to the
value of PGDATESTYLE during connection startup.
</member>
<member>
Running postmaster using the option <option>-o -e</option> to set
dates to the <literal>European</literal> convention.
Note that this affects only some combinations of date styles; for example
the ISO style is not affected by this parameter.
</member>
<member>
Changing variables in
<filename>src/backend/utils/init/globals.c</filename>.
</member>
</simplelist>
</para>
<para>
The variables in <filename>globals.c</filename> which can be changed are:
<simplelist>
<member>
bool EuroDates = false | true
</member>
<member>
int DateStyle = USE_ISO_DATES | USE_POSTGRES_DATES | USE_SQL_DATES | USE_GERMAN_DATES
</member>
</simplelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SERVER_ENCODING</term>
<listitem>
<para>
Sets the multi-byte server encoding
<variablelist>
<varlistentry>
<term><replaceable class="parameter">value</replaceable></term> <listitem>
<para>
Sets the multi-byte server encoding.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
This option is only available if MULTIBYTE support was enabled
during the configure step of building Postgres.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>TIMEZONE</term>
<listitem>
<para>
The possible values for timezone depends on your operating
system. For example on Linux /usr/lib/zoneinfo contains the
database of timezones.
</para>
<para>
Here are some valid values for timezone:
<variablelist>
<varlistentry>
<term>'PST8PDT'</term>
<listitem>
<para>
set the timezone for California
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>'Portugal'</term>
<listitem>
<para>
set time zone for Portugal.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>'Europe/Rome'</term>
<listitem>
<para>
set time zone for Italy.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DEFAULT</term>
<listitem>
<para>
set time zone to your local timezone
(value of the TZ environment variable).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
If an invalid time zone is specified, the time zone
becomes GMT (on most systems anyway).
</para>
<para>
The second syntax shown above, allows one to set the timezone
with a syntax similar to SQL92 <command>SET TIME ZONE</command>. The LOCAL keyword is just an alternate form
of DEFAULT for SQL92 compatibility.
</para>
<para>
If the PGTZ environment variable is set in the frontend
environment of a client based on libpq, libpq will automatically
set TIMEZONE to the value of PGTZ during connection startup.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>TRANSACTION ISOLATION LEVEL</term>
<listitem>
<para>
Sets the isolation level for the current transaction.
<variablelist>
<varlistentry>
<term>READ COMMITTED</term>
<listitem>
<para>
The current transaction queries read only rows committed
before a query began. READ COMMITTED is the default.
</para>
<note>
<para>
<acronym>SQL92</acronym> standard requires
SERIALIZABLE to be the default isolation level.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>SERIALIZABLE</term>
<listitem>
<para>
The current transaction queries read only rows committed
before first DML statement
(<command>SELECT/INSERT/DELETE/UPDATE/FETCH/COPY_TO</command>)
was executed in this transaction.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
There are also several internal or optimization
parameters which can be specified
by the <command>SET</command> command:
<variablelist>
<varlistentry>
<term>RANDOM_PAGE_COST</term>
<listitem>
<para>
Sets the optimizer's estimate of the cost of a nonsequentially
fetched disk page. This is measured as a multiple of the cost
of a sequential page fetch.
<variablelist>
<varlistentry> <term><replaceable class="parameter">float8</replaceable></term>
<listitem>
<para>
Set the cost of a random page access
to the specified floating-point value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>CPU_TUPLE_COST</term>
<listitem>
<para>
Sets the optimizer's estimate of the cost of processing each
tuple during a query. This is measured as a fraction of the cost
of a sequential page fetch.
<variablelist>
<varlistentry>
<term><replaceable class="parameter">float8</replaceable></term>
<listitem>
<para>
Set the cost of per-tuple CPU processing
to the specified floating-point value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>CPU_INDEX_TUPLE_COST</term>
<listitem>
<para>
Sets the optimizer's estimate of the cost of processing each
index tuple during an index scan. This is measured as a fraction
of the cost of a sequential page fetch.
<variablelist>
<varlistentry>
<term><replaceable class="parameter">float8</replaceable></term>
<listitem>
<para>
Set the cost of per-index-tuple CPU processing
to the specified floating-point value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>CPU_OPERATOR_COST</term>
<listitem>
<para>
Sets the optimizer's estimate of the cost of processing each
operator in a WHERE clause. This is measured as a fraction
of the cost of a sequential page fetch.
<variablelist>
<varlistentry>
<term><replaceable class="parameter">float8</replaceable></term> <listitem>
<para>
Set the cost of per-operator CPU processing
to the specified floating-point value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>EFFECTIVE_CACHE_SIZE</term>
<listitem>
<para>
Sets the optimizer's assumption about the effective size of the
disk cache (that is, the portion of the kernel's disk cache that
will be used for Postgres data files). This is measured in disk
pages, which are normally 8Kb apiece.
<variablelist>
<varlistentry>
<term><replaceable class="parameter">float8</replaceable></term>
<listitem>
<para>
Set the assumed cache size
to the specified floating-point value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_SEQSCAN</term>
<listitem>
<para>
Enables or disables the planner's use of sequential scan plan types.
(It's not possible to suppress sequential scans entirely, but turning
this variable OFF discourages the planner from using one if there is
any other method available.)
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of sequential scans (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of sequential scans.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_INDEXSCAN</term> <listitem>
<para>
Enables or disables the planner's use of index scan plan types.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of index scans (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of index scans.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_TIDSCAN</term>
<listitem>
<para>
Enables or disables the planner's use of TID scan plan types.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of TID scans (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of TID scans.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_SORT</term>
<listitem>
<para>
Enables or disables the planner's use of explicit sort steps.
(It's not possible to suppress explicit sorts entirely, but turning
this variable OFF discourages the planner from using one if there is
any other method available.)
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para> enables use of sorts (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of sorts.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_NESTLOOP</term>
<listitem>
<para>
Enables or disables the planner's use of nested-loop join plans.
(It's not possible to suppress nested-loop joins entirely, but turning
this variable OFF discourages the planner from using one if there is
any other method available.)
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of nested-loop joins (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of nested-loop joins.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_MERGEJOIN</term>
<listitem>
<para>
Enables or disables the planner's use of mergejoin plans.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of merge joins (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para> disables use of merge joins.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ENABLE_HASHJOIN</term>
<listitem>
<para>
Enables or disables the planner's use of hashjoin plans.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables use of hash joins (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables use of hash joins.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>GEQO</term>
<listitem>
<para>
Sets the threshold for using the genetic optimizer algorithm.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables the genetic optimizer algorithm
for statements with 11 or more tables.
(This is also the DEFAULT setting.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ON=<replaceable class="parameter">#</replaceable></term>
<listitem>
<para>
Takes an integer argument to enable the genetic optimizer algorithm
for statements with <replaceable class="parameter">#</replaceable>
or more tables in the query.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem> <para>
disables the genetic optimizer algorithm.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
See the chapter on GEQO in the Programmer's Guide
for more information about query optimization.
</para>
<para>
If the PGGEQO environment variable is set in the frontend
environment of a client based on libpq, libpq will automatically
set GEQO to the value of PGGEQO during connection startup.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>KSQO</term>
<listitem>
<para>
<firstterm>Key Set Query Optimizer</firstterm> causes the query
planner to convert queries whose WHERE clause contains many
OR'ed AND clauses (such as "WHERE (a=1 AND b=2) OR (a=2 AND b=3) ...")
into a UNION query. This method can be faster than the default
implementation, but it doesn't necessarily give exactly the same
results, since UNION implicitly adds a SELECT DISTINCT clause to
eliminate identical output rows. KSQO is commonly used when
working with products like <productname>MicroSoft
Access</productname>, which tend to generate queries of this form.
<variablelist>
<varlistentry>
<term>ON</term>
<listitem>
<para>
enables this optimization.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OFF</term>
<listitem>
<para>
disables this optimization (default setting).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DEFAULT</term>
<listitem>
<para>
Equivalent to specifying <command>SET KSQO='OFF'</command>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The KSQO algorithm used to be absolutely essential for queries
with many OR'ed AND clauses, but in Postgres 7.0 and later
the standard planner handles these queries fairly successfully.
</para>
</listitem> </varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="R2-SQL-SET-2">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
SET VARIABLE
</computeroutput></term>
<listitem>
<para>
Message returned if successfully.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
WARN: Bad value for <replaceable class="parameter">variable</replaceable> (<replaceable class="parameter">value</replaceable>)
</computeroutput></term>
<listitem>
<para>
If the command fails to set the specified variable.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</refsynopsisdiv>
<refsect1 id="R1-SQL-SET-1">
<refsect1info>
<date>1998-09-24</date>
</refsect1info>
<title>
Description
</title>
<para>
<command>SET</command> will modify configuration parameters for variable during
a session.
</para>
<para>
Current values can be obtained using <command>SHOW</command>, and values
can be restored to the defaults using <command>RESET</command>.
Parameters and values are case-insensitive. Note that the value
field is always specified as a string, so is enclosed in
single-quotes.
</para>
<para>
<command>SET TIME ZONE</command> changes the session's
default time zone offset.
An SQL-session always begins with an initial default time zone
offset.
The <command>SET TIME ZONE</command> statement is used to change the default
time zone offset for the current SQL session.
</para>
<refsect2 id="R2-SQL-SET-3">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
Notes
</title>
<para>
The <command>SET <replaceable class="parameter">variable</replaceable></command>
statement is a <productname>Postgres</productname> language extension.
</para>
<para>
Refer to <command>SHOW</command> and <command>RESET</command> to
display or reset the current values.
</para>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-SET-2">
<title>
Usage
</title>
<para>
Set the style of date to ISO:
<programlisting>
SET DATESTYLE TO 'ISO';
</programlisting>
Enable GEQO for queries with 4 or more tables:
<programlisting>
SET GEQO ON=4;
</programlisting>
Set GEQO to default:
<programlisting>
SET GEQO = DEFAULT;
</programlisting>
Set the timezone for Berkeley, California:
<programlisting>
SET TIME ZONE 'PST8PDT';
SELECT CURRENT_TIMESTAMP AS today;
today
----------------------
1998-03-31 07:41:21-08
</programlisting>
Set the timezone for Italy:
<programlisting>
SET TIME ZONE 'Europe/Rome';
SELECT CURRENT_TIMESTAMP AS today;
today
----------------------
1998-03-31 17:41:31+02
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-SET-3">
<title>
Compatibility
</title>
<refsect2 id="R2-SQL-SET-4">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
SQL92
</title>
<para>
There is no general
<command>SET <replaceable class="parameter">variable</replaceable></command>
in <acronym>SQL92</acronym> (with the exception of
<command>SET TRANSACTION ISOLATION LEVEL</command>).
The <acronym>SQL92</acronym> syntax for <command>SET TIME ZONE</command>
is slightly different,
allowing only a single integer value for time zone specification:
<synopsis>
SET TIME ZONE { interval_value_expression | LOCAL }
</synopsis>
</para>
</refsect2>
</refsect1>
</refentry>
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